Dynamic Ion Gels from the Complex Coacervation of Oppositely Charged Poly(ionic liquid)s

A cationic poly(ionic liquid) (PIL) with pendent butyl imidazolium cations and free bis(trifluoromethylsulfonyl)imide (TFSI) anions and an anionic PIL with pendent TFSI anions and free 1-butyl-3-methylimidazolium cations are synthesized by postpolymerization chemical modification and reversible addition–fragmentation chain-transfer radical copolymerization, respectively. Upon mixing solutions of these two PILs in acetone with stoichiometric amounts of ion pairs, ionic exchanges induce coacervation and, after solvent evaporation, lead to the formation of a dynamic ion gel (DIG) and the concomitant release of free [1-methyl-3-butyl imidazolium]TFSI ionic liquid (IL). A comparison of thermal (Tg), ion conducting (σDC), and viscoelastic (elastic moduli (G′)) properties for DIGs and their parent polyelectrolytes, as well as extracted and IL-doped DIGs, demonstrates the formation of ionic cross-links and the ability to easily produce polymer electrolytes with enhanced ionic conductivity (σDC up to 4.5 × 10–5 S cm–1 at 30 °C) and higher elastic moduli (G′ up to 4 kPa at 25 °C and 1 rad s–1), making them highly desirable in many electrochemical applications, including supercapacitors, soft robotics, electrochromic devices, sensors, and solar cells.

The characterization of PILs, DIGs and IL release was done by recording of 1 H and 19 F NMR spectra on a Bruker Avance 500 spectrometer in acetone-d6 at 25 °C using a Bruker BBFO 1 H / 109 Ag- 19 F 5mm gradient Z probe.Chemical shifts are reported relative to the acetone-d6 residual proton peaks ( 1 H: δ = 2.05 ppm) for 1 H NMR or relative to an external reference for 19 F NMR ( 19 F: δ = 0.00 ppm, CFCl3).Abbreviations for peak multiplicity are given as follows: s for singlet, d for doublet, t for triplet, q for quartet, p for pentet, dt for doublet of triplets, sext for sextet, m for multiplet and br.s for broad.

Size exclusion chromatography (SEC).
A 1200 Infinity gel permeationsize exclusion chromatograph (SEC, Agilent Technologies) was used to determine number average molar masses (Mn), weight average molar masses (Mw) and chain dispersities (Ð = Mw/Mn) of PILs.The chromatograph was equipped with an integrated IR detector, 0.1 М solution of Li(CF3SO2)2N in DMF was used as an eluent at 50 °C and the flow rate was set as 1.0 mL min -1 .A set of PL PolarGel-M column and a PL PolarGel-M guard column (Agilent Technologies) was applied for analysis of anionic PIL − , while another set of TSKgel G5000-HHR column and TSKgel HHR-H guard column (Tosoh Bioscience) was used to study cationic PIL + .Polymer solutions (4 mg mL -1 ) in 0.1 М solution of Li(CF3SO2)2N in DMF were filtered through 0.20 μm pore size polytetrafluoroethylene (PTFE) filters prior to the measurements.Polymethylmethacrylate standards (EasiVial PMMA, Agilent Technologies, Mp = 550-1568000 g mol -1 ) were used to perform calibration.Differential scanning calorimetry (DSC) measurements were done using a DSC Q200 (TA Instrument) calibrated with an indium standard.The samples were prepared in aluminium hermetic pans and the experiments were conducted under a nitrogen purge of 25 mL min -1 on ca.5-10 mg samples.The sample was first heated to 100 °C at a rate of 10 °C min -1 and isothermally annealed for 1 min.Then, the temperature was decreased to -70 °C at a rate of 10 °C min -1 followed by a second heating to 100 °C at a rate of 10 °C min -1 .The glass transition temperatures (Tg) were measured at the mid-point of the transition (on the second heating cycle) using the TA Thermal Analysis software.
Thermogravimetric analysis (TGA) was performed using a TGA Q500 (TA Instruments).A heating ramp from 30 to 700 °C was applied at 10 °C min -1 under a helium purge of 60 mL min -1 to ca. 5-15 mg samples.The onset weight loss temperature (Tonset) was determined as the point in the TGA curve at which a significant deviation from the horizontal was observed.The resulting temperature was then rounded to the nearest 5°C.
Rheological measurements were performed using a strain-controlled ARES-G2 rheometer (TA Instruments) equipped with an ACS-3 chiller (TA Instruments).Plate-plate disposable aluminum geometries with a diameter of 25 mm were adopted for neat and previously dried PILs.In the case of the dried DIGs, smaller disposable aluminum plates with a diameter of 8 mm were used.Aiming to eliminate the effect of moisture on the viscoelastic properties of the ionic samples, both PILs and DIGs were dried inside a vacuum oven for at least 24 h at 70 C before the measurements.To guarantee a maximum contact between the sample and the geometry, a sufficiently thick layer of 0.4-1.5 mm and an initial positive axial force was applied.Frequency sweep tests were conducted at temperatures ranging from -20 °C to 75 °C from 628 rad s -1 to 0.1 rad s -1 .To determine the linear viscoelastic region of each sample preliminary strain sweep tests were carried out at a fixed frequency of 1 rad s -1 in a range of deformation between 0.01 and 100%.The master curves were built through time-temperature superposition and were referenced at T0 = Tg + 40 K.The corresponding shift factors aT were determined and it was verified that they present a temperature dependence that follows the WLF (Williams-Landel-Ferry) equation S1 4 : The empirical parameters C1 and C2 obtained for T0 = Tg + 40 K are listed in Table S1.
Broadband dielectric spectroscopy (BDS) was employed to measure the effect of temperature on the ionic conductivity of the samples.The measurements were carried out using a high resolution Alpha-Analyzer (Novocontrol GmbH).The sample temperature was controlled under a flow of pure nitrogen gas (Quatro temperature controller) so that any presence of oxygen and moisture in the measuring chamber can be excluded.The thermal stability was better than 0.1 K, with relative variations less than 0.2 K min −1 .For PIL + and PIL − , a solution of 100 mg in acetone (1.5 mL) was deposited drop wise onto a platinum electrode (2 cm in diameter) and the solvent was slowly evaporated under ambient conditions.For DIG, DIGext and DIGIL, ca. 100 mg of previously dried samples at 70 C under vacuum for at least 24 h were directly placed in the platinum electrode with a stainless steel spatula.In order to remove any traces of solvent or water in the samples, a thermal annealing at 70 °C under vacuum for 18 h was carried-out.After the thermal treatment, a second platinum electrode (3 cm in diameter) was placed on top of the sample to build up a measurement cell as a parallel plate capacitor.The sample thickness was controlled by employing 100 m thick Teflon spacers.A further annealing was performed inside the cryostat of the dielectric spectrometer under a flow of pure nitrogen during 3 h at 110 °C.The electrical and dielectric properties were continuously monitored until the equilibrium was reached.Once this equilibration procedure was done, the ionic conductivity measurements were started by measuring the complex conductivity function which is defined by equation S2: The conductivity measurements were carried-out from 10 MHz to 0. ) (eq S3) with DC the ionic conductivity in the high temperature limit, B a fitting parameter related to the activation energy of the ionic conduction, and TVFT the Vogel temperature.The obtained parameters are listed in Table S2.

Determination of the quaternization degree (Q).
The quaternization degree Q was determined by 1

IV. Preparation of dynamic ion gels (DIGs)
General procedure for the preparation of DIGs.Complex coacervates were prepared in stoichiometric conditions regarding ion pairs from cationic PIL + (33 wt%) and anionic PIL − (67 wt%).A solution of PIL + (125 mg, 0.241 mmol of ion pairs) in 1.0 mL of acetone was added dropwise into a solution of PIL − (260 mg, 0.241 mmol of ion pairs) in 1.0 mL of acetone under constant stirring.The formation of a turbid suspension was observed immediately.The resulting suspension, having a total polymer concentration of ~ 20 wt%, was vortex mixed for 1 min to improve mixing quality.Acetone was evaporated under reduced pressure and the resulting DIG was annealed for 24 h at 70 C/1 mm Hg.          mL of fresh acetone were added to the residual coacervate, the suspension was vortex-mixed, centrifuged and the supernatant solution was again removed by decantation.This step was repeated and CFCl3 (34.3 mg, 0.25 mmol) was prepared in acetone-d6 (4.9 mL).Then, a solution of PIL + (7.8 mg, 0.015 mmol of ion pairs) in 400 µL of solution A was added to a solution of PIL − (16.1 mg, 0.015 mmol of ion pairs) in 300 µL of solution A inside an NMR tube.A whitening of the solution was readily observed and after vigorous stirring a white precipitate falling down to the bottom of the tube was formed (Figure 3).After taking a quantitative 19 F NMR spectra (Figure S25), the integrals of the internal calibrant (TFM2B) and the released IL were compared to the calculated mass of released IL (mIL) during DIG formation using equation S11: In the experiment described herein mIL = 5.7 mg.This corresponds to 23.8 wt% of the initial mass S29 of PIL + and PIL − , and it accounts for the generation of 90.8 mol% of free ion pairs according to the initial content of ion pairs in PIL + and PIL − .
H NMR using integral in the region of 4.10-3.40ppm and the following equations: (eq S7) , where   +  is the normalized integral for quaternized polymer PIL + in the region of 4.10-3.40ppm,    is the normalized integral for unmodified Hydrin C2000 in the region of 4.10-3.40ppm,   + ℎ is the theoretical integral for fully quaternized polymer PIL + in the region of 4.10-3.40ppm,   + is the theoretical number of protons corresponding to fully quaternized polymer PIL + in region of 4.10-3.40ppm (Figure S1b, signals 6-9),   is the integral for unmodified Hydrin C2000 in the region of 4.10-3.40ppm,   is the theoretical number of protons corresponding to unmodified Hydrin C2000 in region of 4.10-3.40ppm (Figure S1a, signals 1-5),   +  is the experimentally determined integral for fully quaternized polymer PIL + in the region of 4.10-3.40ppm.

Figure S21 .
Figure S21.Frequency dependence of ionic conductivity σ' measured by BDS from 110 to -50 C for DIGext.

Figure S23 .
Figure S23.Frequency dependence of ionic conductivity σ' measured by BDS from 110 to -50 C for IL.
The extracted acetone layers were united and the solvent was evaporated under reduced pressure.A colorless liquid (94.9 mg) was obtained after drying for 24 h at 70 C/1 mm Hg.This represents 24.6 wt% of the initial mass (i.e., PIL + and PIL − ) and generation of 93.9 mol% of free ion pairs according to the initial content of ion pairs in PIL + and PIL − .1  H NMR (500.2MHz, acetone-d6) δ(ppm): see FigureS24.In-situ quantification of released IL during DIG formation by 19 F NMR spectroscopy.Quantification of released IL was performed by quantitative19  F NMR spectroscopy using a solution of 1,4-bis(trifluoromethyl)benzene (TFM2B, internal calibrant) and trichlorofluoromethane (CFCl3, internal standard, δ = 0.00 ppm) in acetone-d6.To minimize weighing uncertainties, a stock solution (denoted as solution A) of TFM2B (39.2 mg, 0.183 mmol) S11) with IIL the integral of the signal of IL at δ( 19 F) = -78.828ppm, ITFM2B the integral of the signal of TFM2B at δ( 19 F) = -62.679ppm, CTFM2B the concentration of TFM2B in solution A (CTFM2B = 37.35 mM), MIL the molar mass of IL and V the volume of the solution of the PIL + and PIL − mixture.
under anhydrous conditions at 25 or 30 o C; b Temperature at which σDC was measured; c Number-average molar mass; d Number-average molar mass measured for neutral precursor; e Weight-average molar mass.f N.d.=not determined.